In the art of speed sensing, it is known to use an electromagnetic sensor to magnetically sense the proximate presence and absence, in relation to the sensor, of, e.g., teeth on a metallic rotating gear. The sensor generates electrical signal pulses that correspond to the presence of gear teeth as they rotate in proximate vicinity to the sensor. The gear is typically an integral component of, e.g., an automotive or aircraft engine having a plurality of rotating components, the rotating speed of said engine being the overall engine parameter desired to be measured.
In a specific example of a speed sensor for a jet aircraft engine, the electromagnetic sensor must operate in a severe environment. The sensor provides electrical signals through signal wires over a distance of several feet or more to an engine control. The engine control processes the electrical signals to derive speed information therefrom.
Due to the severe environment that the aircraft engine and associated sensing and control components reside in, the aforementioned signal wires carrying the speed information are subject to a number of forms of interference. The most common form of electromagnetic interference is due to lightning, high power RADAR signals or other electrical transmitters. Such interference may be detected as false speed information.
It is known to detect the speed of rotation of the shaft using a fiber optic sensor, as exemplified in U.S Pat. No. 4,746,791 to Forkel. This patent describes a number of embodiments of such a sensor. In only one of the embodiments is an optical source, such as a light emitting diode ("LED"), provided in the housing of the sensor. However, in this embodiment, the LED must be supplied with power from an external power supply. Such external power lead wires are a weight penalty and may be subject to the aforementioned interference effects. Further, the remaining embodiments require optical energy to be transmitted from an external source disposed away from the sensor housing. Thus additional components are required, adding undesirable cost, weight and complexity to the sensor.
In addition, for use with fiber optic sensors, the present day optic signal fibers have a major reliability problem in the optical connector associated therewith. Every time the optic connector is disconnected and reconnected, the engine environment poses the jeopardy of significant or total loss of the optic signals due to contamination. Oil fumes, water vapor, dust and grit can readily block or degrade the optic path which, for a 100 micron diameter fiber is about 0.004 inches across.